Percutaneous transluminal coronary angioplasty (PTCA) is an efficient and effective procedure for treating vascular disease caused by or associated with stenosis of coronary arteries. Typically, a physician fluoroscopically guides a catheter fitted with an expandable balloon, from an entry point at the femoral artery, through a patient's arterial system to the site of the stenoisis or occlusion. The expandable balloon of the catheter is then positioned across the stenosis or occlusion and the balloon is inflated with fluid to dilate the artery and open the obstructed passageway. The dilated artery reestablishes an acceptable blood flow through the artery without resorting to more serious, invasive surgical procedures such as grafts or by-passes.
The PTCA procedure places unique demands on the types of materials needed to fabricate a catheter fitted with an expandable balloon. The physical properties and characteristics of a desirable balloon may result in certain characteristics being balanced against others. For example, a minimum balloon profile is advantageous because it allows the balloon to easily reach and then traverse tight stenosis or occlusions with minimum trauma to arterial vessels. A minimum balloon profile may be achieved by minimizing the wall thickness of the balloon material but thinner walls of the balloon material gives a weaker balloon (compared to a thicker wall) and thus reduces the amount of pressure that can safely be used to inflate the balloon and open a stenosis.
Similarly, very strong thermoplastic materials that are sufficiently strong enough to allow for minimum balloon wall thicknesses tend to be rigid, hard or stiff compared to more elastomeric materials that tend to be flexible, soft and deformable. Using stronger materials may give a minimum profile balloon but the stiffness of the material may be more likely to injure or traumatize the vascular system as the balloon is positioned to and then across a stenosis or occlusion.
In the past, PTCA catheter balloons have been made from polymeric materials which gave balloons that may be broadly categorized into two groups: a) non-distensible balloons and b) distensible balloons. Non-distensible balloons typically inflate to a nominal diameter and then do not significantly stretch or expand beyond that diameter as the pressure is increased. These types of balloons are not stretchable or compliant. See, for example, U.S. Pat. No. 32,983 to Levy which describes a biaxially oriented, polyethylene terephthalate homopolymer (PET) balloon. In comparison, distensible balloons typically inflate to a nominal diameter and then continue to stretch or expand as the inflation pressure is increased until the strength of the balloon material is exceeded and the balloon bursts. Polyvinyl chloride, polyethylenes and homopolymers or copolymers of olefins have been used to make distensible balloons. See, for example, U.S. Pat. No. 4,154,244 to Becker et al. which describes a thermoplastic rubber balloon.
Distensible balloon materials such as polyvinyl chlorides and polyethylenes characteristically have lower tensile strengths compared to non-distensible balloon materials such as PET or polyimides. The comparatively lower tensile strength of distensible balloon may increase the risk of possible balloon failure if the balloon is over inflated.
Distensible balloons having high expansion properties also present the risk that a blood vessel may be damaged or ruptured due to uncontrolled overinflation. Even so, the relatively high expansion properties of distensible balloons compared to non-distensible balloons provides some advantages. The distensible balloon gives the physician some margin of error in matching a specifically sized balloon with the size of the vessel at the stenosis site. At least in theory, the physician will select a balloon which has the same inflated diameter as the finally dilated artery. In practice; however, assessment of the artery's size can be miscalculated and the greater expansion of a distensible balloon allows the physician to obtain a correct dilatation diameter by using higher inflation pressures (provided, of course, that the balloon material may handle such higher pressures).
Non-distensible balloon materials characteristically have much higher tensile strengths than distensible balloon materials. The higher tensile strengths of nondistensible balloon materials are generally a result of orienting the balloon material during manufacture of the balloon. The orientation process unavoidably imparts stresses and varying degrees of crystallinity or nonhomogeneity in the balloon material which, if ignored, gives an undesirable balloon. PET is a sensitive, unforgiving material when it is processed. During the stretch blow molding process typically used to form PET balloons, the risk exists that thin walled PET balloons will have nonhomogeneous regions in the balloon wall. This nonhomogeneity may result in the formation of pinholes and other weakening of the wall which can lead to balloon failure when it is inflated under pressure. Pinholes are particularly disadvantageous because if the pinhole forms when the balloons is under elevated pressure, a high velocity jet of inflation fluid may be emitted which can cause arterial dissection. The nonhomogeneity of PET materials may be compensated for by using thicker balloon walls but thicker walls may not be preferred. Further, PET balloon have been found to be quite fragile and may be easily damaged during routine handling. The non-distensible properties of PET balloon also require that a physician has to withdraw and replace a balloon which proves to be smaller than needed to fully dilate the artery. Finally, PET balloons have been found to develop extensive wrinkles when the balloon is sterilized. These undesired wrinkles may inhibit the easy advancement of the catheter through the arterial system.
Furthermore, PET balloon materials do not readily take a fold or a crease. As such when these balloons are collapsed in a deflated state the collapsed balloon flattens and provides an undesired "winged" profile. The phenomenon of "winging" results when the flat, lateral portions of the deflated balloon project laterally outward beyond the rest of the catheter. A "winged" balloon presents a profile having rigid edges which has a much higher likelihood of injuring the arterial system during placement of the balloon.
In addition to PET, other types of materials have been used to produce non-distensible balloons are reported. See, for example, U.S. Pat. Nos. 4,938,676 and 4,906,244 that report using a biaxially oriented nylon or polyamide material, U.S. Pat. Nos. 4,884,573 and 4,952,357 that report using a polyimide material and U.S. Pat. No. 4,950,239 that reports using a polyurethane material.
In spite of extensive ongoing efforts to produce an "ultimate" catheter balloon, no single material has been found to be overwhelmingly satisfactory for balloons needed to perform PTCA.
There is clearly a continuing need in this field for a catheter balloon made from a polymer material having properties which are optimized for PTCA.